1. Field of Invention
This invention pertains to the recording of information on magnetic tape using magnetic tape drives, and particularly to a tracking technique enabling reading of non-native formatted stripes.
2. Related Art and Other Considerations
For decades magnetic tape has been employed as a medium for storing information. Devices known as tape drives, tape decks, or tape recorders perform input and output operations, e.g., reading and recording operations, by transducing information from and to the tape. Information to be stored on a tape is obtained from a host device such as a computer. The information is transmitted from the host over a special connection or bus, e.g., SCSI bus, to the tape drive. Typically the tape drive has a buffer memory for temporarily storing information obtained from the host which the drive is not quite ready to record on the tape. When the information is ready to be recorded, the information passes through a write channel of the tape drive to a write head. The head has gaps or other appropriate elements thereon which form magnetic flux transitions on the tape in the recording operation.
A reading operation for a tape drive is essentially the reverse of the recording operation. In the reading operation, the read head detects magnetic flux transitions on the tape to obtain a read signal, processes the signal in read circuitry, stores information ascertained from the read circuitry in the buffer, and ultimately transmits the information to a utilization device, e.g., the host, over the bus which connects the host and the tape drive.
One type of magnetic recording is helical scan recording. In helical scan recording, one or more head(s) are mounted on a rotating drum. The tape is transported past a portion of the periphery of the rotating drum so that, as the head on the drum contacts the tape, a stripe or track is recorded on the tape at an angle to the direction in which the tape is transported. The tape is transported through a tape path, which includes around the periphery of the drum, from a tape supply reel to a tape take-up reel. In some magnetic tape drives, a capstan is utilized to impart linear velocity to the tape. By contrast, a capstanless helical scan tape drive is shown in U.S. Pat. No. 5,602,694 for CAPSTANLESS HELICAL DRIVE SYSTEM to Robert J. Miles and James Zweighaft, which is incorporated herein by reference.
In order for the head(s) of the tape drive to follow the helically recorded stripes, a stripe tracking system must be provided. Many previous helical scan tape drives have tracking systems based on special recorded signals intermixed with data and formatting signals. These are usually referred to as "pilot" or "tracking" signals. The tracking schemes are all based on reading the amplitude of these signals, which varies with the position of the read head relative to the pilot signal. When a read head sweeps directly over a pilot signal, a large amplitude is reproduced. If the read head is off center so that it does not pass completely over the pilot signal, a lesser amplitude is reproduced.
The pilot signals can be of a frequency within the range of data, or at a lower or higher frequency. Low frequency signals may be read by a head using a different gap angle than the track itself. Such a situation is well known to those skilled in the art as `off azimuth` reading. Some systems employ a dedicated servo head whose sole function is to read the pilot signal amplitude.
Various methods are employed to determine the optimal time to sample the amplitude, usually based on a time delay from a synchronizing signal on the tape or an index pulse on the rotating scanner.
As technology advances, particularly the science of manufacturing heads with smaller gaps, helical stripes are becoming narrower. While advantageously generally, the narrowing of gaps and stripes presents a problem when using a narrow gapped-head to read a broader stripe recorded by an older tape drive.
As an example of the foregoing, a tape drive manufactured by Exabyte Corporation as the Mammoth.sunburst..TM. tape drive records data tracks that are 11.5 .mu.m wide. To do this, it employs two write heads which are spaced apart by that distance. For playback, there are also two read heads, also spaced 11.5 .mu.m apart. In order to allow for some error in the position of the read heads on playback, the read heads are made somewhat wider than the track pitch. Signal pickup from the adjacent tracks are greatly attenuated by use of the well known dual azimuth scheme. In the dual azimuth scheme, the odd and even tracks are written using different gap angles.
Difficulties arise when the Mammoth.sunburst..TM. drive described above attempts to read other tracks formats, which can vary not only in track width, but also in frequency and spacing of pilot signals, use of a guard band and numerous other factors. One particular case of interest is occurs when the Mammoth.sunburst..TM. drive attempts to read a format of tape recorded by an EXB.RTM.-8500 model tape drive, manufactured by Exabyte Corporation. Information regarding the EXB.RTM.-8500 model tape drive is disclosed e.g, in U.S. Pat. Nos. 5,142,422; 5,065,261; and 5,068,757, all of which are incorporated herein by reference.
Because the track width on the EXB.RTM.-8500 format is different than the spacing between the two Mammoth read heads, it is not possible to have both read heads centered over two 8500 tracks simultaneously. The tracks of the EXB.RTM.-8500 are nominally 15.5 um wide with no guard band between them, while the Mammoth.sunburst..TM. read heads are separated by 11.5 um.
What is needed therefore, and an object of the present invention, is a method and apparatus for enabling a helical scan tape drive with narrow head gaps to read stripes recorded differently, e.g, with broader head gaps.